Radio frequency (RF) resonant circuits are fundamental building blocks in a wide variety of electronic applications, such as wireless communications. Examples include their use in frequency up/down-converters, IQ modulators, and variable gain amplifiers. In many cases, the resonant circuits must exhibit high signal to noise ratio (SNR), high linearity, and low phase noise.
As one representative example, FIG. 18 depicts the basic functionality of an RF transmitter 10, such as may be found in a mobile communication system handset. The transmitter 10 includes one or more low-pass filters 12 and an IQ modulator 14 receiving transmission frequency signals from an IQ clock generator and driver circuit 16. The modulator up-converts the baseband signals into radio frequency (RF) by a direct up-conversion where the baseband frequency signals are multiplied by a local oscillator (LO) signal. The IQ modulated signal is filtered by a band-pass filter and variable gain amplifier (VGA) 18, which provides variable gain as required by the transmitter signal path. A surface acoustic wave (SAW) filter 20 further filters the RF signal before it is amplified for transmission by a power amplifier 22.
SAW filters 20 are effective in suppressing spurious emission outside the SAW pass band, as well as noise arising from transmitter leaking to other circuits, such as a receiver. However, mobile devices operable in multiple frequency bands, such as those compatible with different communication standards, require separate SAW filters 20 for each operating frequency. This requires an increase in the number of electronic parts in the device, raising cost, increasing power consumption, and defeating miniaturization. Replacing the SAW filter 20 in the transmitter 10 would allow for a more compact and lower cost design. However, there are several challenges to be met in replacing SAW filters 20.
The noise spectrum at a transmitter 10 can extend into the receiver band, and due to limited isolation of an antenna duplexer, the transmitted signal also creates a leakage at the receiver front-end, causing degradation to the receiver noise figure. To remove the SAW filter 20, the IQ modulator 14 and VGA 18 must have enough SNR, and must not generate high levels of noise in the receiver frequency band(s). To improve SNR, the signal level must be large, which implies high power consumption. Theoretically, twice the power consumption can improve SNR 3 dB, but in practice only 2˜2.5 dB can be expected.
The spurious emission in other radio frequency bands and channels must be low enough to meet the spectrum mask requirements. This calls for high linearity in the IQ modulator 14 and VGA 18, and a high quality baseband signal. Non-linearity in IQ modulator 14 and VGA 18 will create new frequency interference products, which may influence other radio devices nearby. Digital baseband signals are converted to analog in, or prior to, the IQ modulator 14, and the re-construction error in this conversion must be very low.
The phase noise in local clocks must be very low; otherwise, the phase noise will be up-converted into radio frequency noise. This requires a phase locked loop (PLL) with very low phase noise, which is very difficult to design. This is particularly true for a single-chip RF transceiver design, where the interference between different functional blocks cannot be ignored.
It is thus apparent that replacing the SAW filters 20 in a mobile communication transmitter 10 is a daunting task. Several approaches are known in the art. Ahmad Mirzaei and Hooman Darabi propose one such approach in the paper, “A Low-Power WCDMA Transmitter with an Integrated Notch Filter,” published in the ISSCC-2008, at p. 212-213 (2008). In this approach, a feedback notch filter in the transmitter 10 path is intended to have a notch filter at the frequency of the receiver band, which provides about 19 dB suppression in frequency transfer function. However, the disturbance to the desired transmitter signal due to the insertion of the notch filter is a serious problem. Another issue is that the notch filter itself introduces noise into the mixed RF output. Therefore, the real benefit of this approach is questionable.
Another known approach to replacing the SAW filter 20 is to replace the traditional analog IQ modulator 14 with a direct digital RF modulator, as described in the paper by Andras. P. et al., “A fully digital 65 nm CMOS transmitter for the 2.4-to-2.7 GHz WiFi/WiMax bands using 5.4 GHz ΔΣRF DACs,” published in the ISSCC-2008, at p. 360-361 (2008). The direct digital RF modulator places a very high demand on digital-to-analog converter (DAC) for very high bit resolution and high sampling frequency. The spurious emission is still a problem, the published results to date do not appear sufficient to solve the problems in a SAW-less solution.
It is known in the art to form a resonant circuit from an inductive load, such as a balun or inductor, with a capacitor. This circuit is known in the art as an LC tank. By making the capacitor variable, the LC tank can be used to implement band pass filter that can be tuned to the desired frequency selectivity. The filter frequency response depends primarily on the Q of the passive inductive devices, which for an on-chip inductor may range from 5 to 20. Both the capacitor and inductor will exhibit errors due to process variation. For example, a relative capacitance error about +/−20% is quite common. Due to this wide variation, few designers utilize resonant LC tanks in their filter designs. LC tanks are more widely used in PLL applications, where the output frequency of an oscillator is the resonate frequency, which has a fixed m/n ratio with a reference clock frequency. In this application, the LC tank remains tuned at the desired resonate frequency as long as the loop is locked. To guarantee loop locking, however, an accurate setting is required. Different tricks are employed to achieve this, and PLLs based on LC tanks usually take a relatively longer time to lock than other designs.